1. Field of Invention
The present invention in general relates to a multiple-beam holographic optical pick-up head. More particularly, this invention combines the advantages of the multiple-beam optical pick-up head and the holographic optical pick-up head and can be the data retrieval pick-up head for the optical recording media.
2. Related Art
The data reading and writing on optical recording media such as the CD, CD-R and DVD are done with the help of an optical pick-up head. When writing, the laser beam converges on the data storage surface of the optical disk and changes its physical property; while when reading, the laser beam first converges on the data storage surface of the disk, then gets reflected and absorbed by a photodetector. This beam containing data signals is therein converted into the corresponding electrical signals.
Conventional optical disk drives use only a single beam to access data on the disk. Their speeds in reading/writing can only be increased by increasing the rotational speed of the motor, which is the way most high-speed optical disk drives adopt. However, the motor rotational speed has its own limit, this method is thus imperfect.
Multiple-beam reading/writing uses a special grating to split a laser beam into a plurality of separate laser beams that simultaneously accessing data in different tracks of the optical disk. With special control and a decoding circuit, data in different tracks are then recombined together. Therefore, this method can greatly increase the reading/writing speed of an optical disk drive.
FIG. 1 shows the optical path of an optical pick-up head with multiple beams.
A light source 101 generates an incident light, which is split by a grating 102 into a plurality of beams. After passing through a beam splitter 103, a collimator lens 104, and an objective lens 105, the beams reach the optical disk surface and am reflected. The reflected beams then pass through the objective lens 105, the collimator lens 104 and are deflected by the beam splitter 103. Finally, after passing through another grating 106, the reflected beams are absorbed by a photodetector 107.
A conventional optical pick-up head comprises a beam splitter, a laser diode, a cylindrical lens, a photodetector, and etc. Since the elements are conventional separate ones, the package volume and weight of the elements are larger.
A holographic optical pick-up head replace the optical devices such as beam splitter and the cylindrical lens of a pick-up head in the prior art with a piece of tiny hologram. It shortens the distance between the laser diode and the photodetector. Therefore, all devices can be encapsulated within a tiny package called the holographic laser module.
FIGS. 2(a) and 2(b) show a conventional optical pick-up head and a holographic optical pick-up head, respectively.
The components described in FIGS. 2(a) and 2(b) are all well known in the art. They include: light source (201, 211), grating (207, 212), prism (203), collimator lens (204, 213), objective lens (205, 214), reflector (206, 215), another grating (202), and photodetector (208, 216).
Currently, holographic laser modules are very popular in single beam pick-up heads. But there is no multiple-beam optical pick-up head using the holographic laser module. The main reason is that the laser beam usage efficiency of the holographic optical pick-up head is low. For a single beam pick-up head, the energy efficiency of one that uses a holographic laser module is only {fraction (1/12)} of that of a conventional one. The multiple-beam pick-up head further splits one laser beam into a plurality of separate beams and thus diverse the laser beam energy. If one further combines both, the energy efficiency is even lower. Since the energy that reaches the photodetector decreases in such systems, the associated photoelectric current after photoelectric conversion is so weak that the S/N ratio is small. Then the gain of the I-to-V preamplifier has to be raised to meet the need. However, the frequency band of an amplifier is inversely proportional to the gain. Therefore, increasing the gain of the preamplifier will decrease the allowed frequency band of the pick-up head and thus limit the data access speed.
To sum up, the multiple-beam optical pick-up head has the advantage of greatly increasing the access speed of the optical disk drive, the holographic optical pick-up head has the merits of a small volume, easy to design and produce. If one wants to combine both, i.e., to have a holographic pick-up head with multiple beams, how to increase the laser energy efficiency is an important technical issue to be solved.
It is a principal object of the present invention to provide a multiple-beam holographic optical pick-up head that can increase the laser beam usage efficiency.
The disclosed technology uses a polarized diffractive optical element to diffract laser light polarized in a specific direction while leaving the laser light polarized in the other direction unaffected at all. Two polarized diffractive optical elements are provided on the optical path of the system. One is a multiple-beam grating which can diffract one incident beam into several beams, but let reflective beams to completely pass through. The other is a hologram which allows the incident light to completely pass through without diffraction and diffracts the reflective beams. A quarterwave plate is provided on the optical path of the pick-up head for changing the polarization directions of the incident light and reflected light. Since both the incident beams and reflected beams experience diffraction once only, there would not be such problems as low efficiency. The multiple-beam holographic optical pick-up head is formed by combining both the multiple-beam optical pick-up head and the holographic optical pick-up head, and thus has the advantages of both systems.